1. Field of the Invention
The present invention relates to a process control apparatus that can precisely control a process in accordance with the state or charasterictics of the process.
2. Description of the Prior Art
In process controlling the temperature, flow, pressure, and the like of a plant, the PID control method is widely employed. In particular, for steady states, the operation signals from the PID control apparatus are stable and the PID control performance has been implemented extensively for a long time with skilled engineers. On the other hand, in order to appropriately execute control with a process control system that employs the PID method, it is necessary to adjust, or tune, suitably the control constants of the PID control apparatus in compliance with the characteristics of the process. However, the tuning for an appropriate PID control, at all times, of a process in which the characteristics vary frequently due to external disturbances, load, and so forth is a difficult job which is a serious burden to the operator. For this reason, there has been proposed a sampling PID control apparatus (described, for example, in Japanese patent No. 90704/1982) which is equipped with an auto-tuning mechanism for automating the tuning work for the PID control. As an improvement over this, there has been a demand for an apparatus that can provide a satisfactory control performance even for cases where the process characteristics vary frequently, such as the case where the process chasacteristics change during the period in which the characteristics of the process are being identified.
On the one hand, it has been known, for processes in which characteristics vary frequently, that an adaptive process control that employes the minimum variance control method, deadbeat control method, pole assignment control method, or the like is effective. Description of these control methods can be found, for example, in H. Kurz et a., "Development, Comparison and Application of Various Parameter--Adaptive Digital Control Algorithms" (Preceeding of the 7th IFAC Congress, Helsinki (1978), pp. 443-452) or in K. J. Astrom et al., "On Self Tuning Regulators" (Automatica, 1973, No. 9, pp. 185-199). On the other hand, there are disadvantages in these control methods that they may shorten the life of control equipments and the like due to the tendency of greatly modifying the operation signals in controlling the process, even in steady states where these are no changes in process characteristics. Furthermore, in contrast to the number of three, at the most, of the control constants for the PID control apparatus, the corresponding number of control constants for these methods may exceed ten, making the mannal tuning of the apparatus extremely difficult. These factors represent the obstacles in introducing the adaptive control methods into the practical scheme of process control.